1. Field of the Invention
This invention relates to an exhaust gas recirculation system (EGR) for reducing the amount of NOx contained in exhaust gas of an internal combustion engine equipped, for example, in an automotive vehicle, and more particularly to an exhaust gas recirculation system (hereinafter, referred to as an EGR system) capable of judging the abnormality of the operation of EGR system.
2. Related Art
Malfunction of the EGR system leads to an increase of NOx in exhaust gas. To prevent such an increase of harmful emission, an EGR abnormality judging system having been already developed detects the abnormality of the EGR system and notifies it of the driver by means of an appropriate alarming device. This kind of system, for example disclosed in Unexamined Japanese Patent Application No. 62-51746 published in 1987, controls the open/close state of an EGR valve installed in an exhaust gas recirculation passage to open and close in a repetitive manner (e.g. open.fwdarw.close.fwdarw.open), detects a first intake air amount (or intake air pressure) in a valve-opening condition and a second intake air amount (or intake air pressure) in a valve-closing condition, and compares the difference of the first and second intake air amounts with an abnormality judging value, thereby judging presence of abnormality of the EGR system. This judgement is based on the fact that, irrespective of open/close control of the EGR valve, the change of the intake air amount (or intake air pressure) is small once the EGR valve was damaged.
However, this system has a disadvantage in that it is not so reliable when the EGR rate is low, because the absolute change of intake air amount (or intake air pressure) in such a low-EGR-rate condition is too small to guarantee the accuracy of the abnormality judgement.